fix the first of the issues: too strict test requirement, see #2950

LARS may execute recurring tasks a little too early on occasion, which
is a direct consequence of it trying to keep them running more regularly
and also allowing them to run at 1/tick rate.
make sure that InterruptExceptions are not swallowed, see #2963

also remove an old work-around in CallingThreadDispatcherModelSpec and
describe the rules for interrupting in the testing docs
Merge pull request #1068 from akka/wip-2950-LARS-∂π

fix the first of the issues: too strict test requirement, see #2950